1. Field
The present disclosure relates generally to an improved data processing system and in particular to a method and apparatus for processing image data. Still more particularly, the present disclosure relates to a method, apparatus, and computer usable program code for focusing an image using data generated by synthetic aperture radar.
2. Background
Synthetic aperture radar (SAR) is a form of radar in which a beam is generated that may be used for applications such as, for example, remote sensing and/or mapping. Synthetic aperture radar systems are designed to produce high resolution images in the azimuth direction as well as in the range direction without the need of a large antenna. This type of resolution is achieved with a smaller antenna by integrating a collected array of pulses generated by the synthetic aperture radar system.
Radar antennas may be used with various types of mobile platforms. In one application, a radar antenna may be attached to an aircraft. In another application, the radar antenna may be attached to a spacecraft or satellite.
A single pulse or signal may be radiated from the antenna that may have different dimensions in the horizontal and vertical directions to achieve desired beam width. Oftentimes, the signal radiated from the antenna may illuminate the terrain below the mobile platform outwards towards the horizon. The amplitude and phase of the signal returning from a given location on the ground may be recorded.
Further, the mobile platform may transmit a series of pulses as the mobile platform travels. The responses detected from these pulses may be combined. In other words, a series of different observations or phases may be combined through different processes to generate an image. This image may be, for example, a map of reflectivity including both amplitude and phase. In these examples, phase information is typically discarded. The amplitude information may contain information about ground cover in the same manner as a black and white picture.
Phase error may occur when receiving responses to pulses to form an image. Spatially variant phase error is a phase error that varies with target position for a particular scene. Spatially variant phase error is a phase error that may change with respect to different targets located within a scene or changes in elevation in the target itself. A scene is an area of interest in which radar data is obtained in these examples. A target may be, for example, a vehicle, a ravine, a mountain, or some other object or area of interest.
Spatially variant phase error may be caused by a number of different sources. For example, non-planar motion by the system in which the synthetic aperture radar is contained may cause phase error. Further, targets not in the focus plane also may cause phase error. Changes in terrain elevation of the target may be another source of phase error. These types of phase error are referred to as spatially variant phase errors. Spatially variant phase error results in images that are not uniformly focused across a scene.
In currently available synthetic aperture radar systems, autofocus techniques are used to focus images generated by synthetic aperture radar systems. Autofocus techniques estimate and correct phase error that is common to all targets. However, as the image resolution increases, the phase error may vary with the target location in the scene. As a result, a uniformly focused image may not be produced through this type of approach.
Another approach may involve using terrain elevation data for use in correcting phase error. Although this type of process may be efficient, implementing or actually performing the process becomes more difficult with the need for terrain data. For example, with this type of technique, elevation data for the target area is needed. Further, other techniques may be focused only on correcting for phase error caused by non-planar motion and do not take into account phase error caused by spatial variations.
Therefore, it would be advantageous to have a method, apparatus, and computer program product that overcomes the problems described above.